Memory array with self-aligned epitaxially grown memory elements and annular FET

ABSTRACT

A system and method for fabricating a memory array device. An example memory array device includes a plurality of memory cells, each including a FET over a substrate and a memory element over the FET. Each memory element includes a plurality of epitaxially grown memory element layers. The memory array device includes a plurality of gate conductors configured a first axis, in parallel. Each gate conductor laterally surrounds a plurality of FETs of the memory cells along the first axis. The memory array device includes a plurality of bit lines configured along a second axis, in parallel, and electrically coupled to a plurality of memory elements along the second axis. Embodiments of the memory array preserve alignment of crystal lattices beginning from the bottom layers in the FET up to the top active layers in memory element, thus preserving crystal lattice alignment between transistor and memory element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §120 to U.S. patentapplication Ser. No. 13/834,998 filed Mar. 15, 2013, the entire text ofwhich is specifically incorporated by reference herein.

BACKGROUND OF INVENTION

The present invention relates to memory array devices. Moreparticularly, the present invention relates to the structure andfabrication of memory element layers in non-volatile memory arraydevices.

Typical non-volatile memory array devices consist of memory cellsfabricated on semiconductor substrates. The memory cells in such memoryarray devices generally consist of memory elements and field effecttransistors (FET) electrically coupled to word-lines and bit-lines.

Spin Torque Transfer (STT) Magnetoresistive Random Access Memory (MRAM)is an attractive emerging memory technology, offering non-volatility,high performance and high endurance. The STT MRAM memory cell typicallyconsists of a Magnetic Tunnel Junction (MTJ) in series with aword-line-gated field effect transistor, and with a bit-line at one orboth ends. If only one end of the cell is connected to a BL, the otheris connected to a mid-level voltage (Vmid). Conventionally, the FET isconstructed within a silicon substrate and the MTJ is constructedbetween two subsequent wiring levels. The contacts, landing pads, andwiring conflicts associated with this structure decrease density, yieldand reliability. Additionally, the trend of increasing density presentsconflicts with performance, yield and reliability of the memoryelements.

SUMMARY OF INVENTION

An aspect of the present invention is a memory array device comprising aplurality of memory cells. Each memory cell includes a FET on top of asubstrate and a memory element electrically coupled to the FET. Eachmemory element includes a plurality of memory element layers, which areeach epitaxially grown. The memory array device also includes aplurality of gate conductors configured along a first axis, parallel toone another. Each gate conductor laterally surrounds a plurality of FETsalong the first axis. The memory array further includes a plurality ofbit lines configured along a second axis, parallel to one another andperpendicular to the gate conductors. Each bit line is electricallycoupled to a plurality of memory elements along the second axis.

Another aspect of the invention is a method for fabricating a memoryarray device. The method includes forming a plurality of FET layers overa substrate. The method also includes forming a plurality of memoryelement layers over the FET layers, such that each memory element layeris epitaxially grown. The method further includes forming a plurality ofmemory cell pillars by etching through portions of the memory elementlayers and FET layers, such that each memory cell pillar includes a FETand a memory element. The method also includes forming a plurality ofgate conductors configured along a first axis, parallel to one another.Each gate conductor laterally surrounds a plurality of FETs along thefirst axis. The method includes forming a plurality of bit linesconfigured along a second axis, parallel to one another. Each bit lineis electrically coupled to a plurality of memory elements along thesecond axis, perpendicular to the first axis.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a cross-sectional view of a memory array device, inaccordance with one embodiment of the present invention.

FIG. 2 depicts a cross-sectional view of a memory array device, inaccordance with one embodiment of the present invention.

FIG. 3 depicts a top-down view of a line-space patterned memory arraydevice, in accordance with one embodiment of the present invention.

FIG. 4 depicts a top-down view of a fused-circle pattern memory arraydevice, in accordance with one embodiment of the present invention.

FIG. 5 depicts a top-down view of a spaced-circle memory array device,in accordance with one embodiment of the present invention.

FIGS. 6 and 7 are flowcharts depicting a method for fabricating a memoryarray device, in accordance with one embodiment of the presentinvention.

FIG. 8 is a flowchart depicting different methods for forming the gateconductors, in accordance to embodiments of the present invention.

FIGS. 9-11 depict intermediary steps throughout fabrication of a memoryarray device, in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention is described with reference to embodiments of theinvention, but shall not be limited to the referenced embodiments.Throughout the description of the present invention, references are madeto FIGS. 1 through 11.

It is initially noted that the environments described below and depictedin the figures are for the purpose of illustrating suggested embodimentsof the present invention. Thus, the present invention should not beconstrued as limited to the environment configurations shown anddiscussed herein. Therefore, the present invention encompassesalternative orientations and configurations of the suggestedembodiments.

Additionally, relative terms, such as “top”, “down”, “above”, and “over”are employed with respects to other elements in the describedembodiments and figures. Such terms are meant only to describe thereferenced embodiments. Likewise, the figures include references to afirst axis and a second axis from the claimed elements, however, thespirit of the invention is not limited by the orientations of such axisin the provided embodiments. Therefore, the present inventionencompasses alternative orientations and configurations of the suggestedembodiments.

Embodiments of the present invention provide possible configurations fora memory array device, and possible methods for fabricating such amemory array device. For high density MRAM memory array, all layers inmagnetic tunnel junctions need to be atomically flat and have uniformthickness across all the devices. Currently, MRAM cells are typicallybuilt on top of all CMOS layers and, as a result, magnetic tunneljunctions have polycrystalline layers with random orientations ofcrystals. This causes a large cell to cell variation in both electricand magnetic properties of individual cells. All these cell to cellvariations can be overcome by growing a MTJ stack epitaxially layer bylayer on mono-crystalline semiconductor surface (Si, Ge). Additionalepitaxial seed layers or etch stop layers can be used for variousproposes for easier manufacturing.

FIG. 1 depicts a cross-sectional view of a memory array device 100 inaccordance with one embodiment of the present invention. The memoryarray device 100 includes a plurality of memory cells 101. Each memorycell 101 includes a field effect transistor (FET) 102 and a memoryelement 108. The FET 102 includes a plurality of FET layers 104 over asubstrate 103. In the preferred embodiment, the FET layers 104 are eachepitaxially grown. The FET layers 104 include alternating layers ofn-type doped silicon and p-type silicon.

The memory element 108 is electrically coupled to the FET 102. Eachmemory element 108 includes a plurality of epitaxially grown memoryelement layers. In some embodiments, the memory element 108 is a SpinTorque Transfer (STT) Magnetoresistive Random-Access Memory (MRAM) cell.In such embodiments, the memory element 108 includes a magnetic tunneljunction (MTJ), which includes a free-magnetic layer 110, a tunnelbarrier 112, and a fixed-magnetic layer 114. In some embodiments, thecrystalline structure of the free-magnetic layer 110 is alignedepitaxially to the FET layer below. The free-magnetic layer 110 andfixed-magnetic layer 114 may be comprised of ferromagnetic materials,cobalt iron boron (CoFeB), cobalt nickel (CoNi), iron boron (FeB), etc.The fixed-magnetic layer 114 may also include antiferromagneticmaterials, such as cobalt iron (CoFe), iron nickel (FeNi), nickel oxide(NiO), etc. The tunnel barrier 112 may be comprised of insulating orsemiconducting materials, such as magnesium oxide (MgO), aluminum oxide(AlO), hafnium oxide (HfO), iron oxide (FeO), bismuth iron oxide(BiFeO), etc.

Embodiments of the present invention preserve alignment of crystallattices beginning form the bottom layers in the transistor 102 up tothe top active layers in memory element, thus preserving crystal latticealignment between the transistor 102 and the memory element 108. Such astructure is in short defined as heteroepitaxial. Embodiments of thememory array preserve alignment of crystal lattices beginning from thebottom layers in the FET up to the top active layers in memory element,allowing for high yielding gigabit memory arrays composed of almostidentical electrically and magnetically cells.

In this embodiment, a seed layer 109 exists between the FET 102 and thememory element 108. The seed layer 109 includes many sub-layers, somesub-layers may only be atomically thick and not continuous. For example,a first sub-layer may provide ohmic contact, a second sub-layer maycompensate for crystal lattice mismatch and a third sub-layer maypromote perpendicular anisotropy in the free-magnetic layer 110. Thesub-layers may be comprised of many materials including but not limitedto aluminum (Al), magnesium oxide (MgO), nickel silicide (NiSi), andcobalt silicide (CoSi). Additionally, there may be several monolayers inbetween the sub-layers. The monolayers configured to promote layer bylayer growth of the whole structure.

In this embodiment, the free-magnetic layer 110 is situated above theFET 102, the tunnel barrier 112 over the free-magnetic layer 110, andthe fixed-magnetic layer 114 situated above the tunnel barrier 112.However, it should be noted that alternate embodiments may exist whereinthe free-magnetic layer 110 is situated over the tunnel barrier 112, thetunnel barrier 112 over the fixed-magnetic layer 114, and thefixed-magnetic layer 114 situated over the FET 102.

The memory array device 100 also includes a plurality of gate conductors106 configured along a first axis (running in and out of FIG. 1),parallel to one another. In this embodiment, the gate conductors 106 areutilized as the conductive channels of the word lines. Each gateconductor 106 is self-aligned to laterally surround a plurality of FET102 of the memory cells 101 along the first axis. As such, each FET 102is configured as an annular FET. One skilled in the arts would recognizethat the gate conductors 106 may include any number of conductivematerials. As such, the gate conductors 106 provide electrical couplingof adjacent memory cells along the first axis. It should be noted that athin gate oxide may be situated between the gate conductor 106 and theFET 102.

The memory array device 100 also includes a plurality of bit lines 116configured along a second axis (running horizontally across FIG. 1),parallel to one another. Each bit line 116 is electrically coupled to aplurality of memory elements 108 of the memory cells along the secondaxis. The second axis being perpendicular to the first axis.

Additionally, the memory array device 100 may include a metallic caplayer 115 between the memory element 108 and the bit lines 116. Themetallic cap layer 115 configured to protect the memory element 108 fromdamage during fabrication. One skilled in the arts would recognize thatthe metallic cap layer 115 may be comprised of any number of conductingmaterials.

The space between the memory cells 101 is filled with a filler layer118. The filler layer 118 may be comprised of any number of insulatingmaterials. In this embodiment, the space between the gate conductors 106is substantially less than the space between the bit lines 116.

FIG. 2 depicts a cross-sectional view of a memory array device 200, inaccordance to one embodiment of the present invention. This embodimentincludes an encapsulation layer 204 surrounding the memory element 108.The encapsulation layer 204 may be configured to protect the memoryelement 108 during the etch processes. Additionally, the encapsulationlayer 204 may be configured to decrease the capacitance between thememory element 108 and the gate conductors 106. In some embodiments, thediameters of the FETs 102 are substantially greater than the diametersof the memory elements 108.

FIG. 3 depicts a top-down view of a line-space patterned memory arraydevice, in accordance with one embodiment of the present invention. Inthis embodiment, each of the gate conductors 106 is patterned as arectangle with a plurality of holes surrounding the memory cells 101along the first axis (running vertically in FIG. 3).

FIG. 4 depicts a top-down view of a fused-circle patterned memory arraydevice, in accordance with one embodiment of the present invention. Inthis embodiment, each of the gate conductors 106 is patterned asoverlapping rings surrounding the memory cells 101 the first axis(running vertically in FIG. 4).

FIG. 5 depicts a top-down view of a spaced-circle patterned memory arraydevice, in accordance with one embodiment of the present invention. Inthis embodiment, each of the gate conductors 106 is patterned asalternating ring portions and rectangular portions along the first axis(running vertically in FIG. 5), each of the ring portions surround oneof the memory cells 101.

FIGS. 6 and 7 are flowcharts depicting a method for fabricating a memoryarray device, in accordance with one embodiment of the presentinvention. The method begins with forming step 602. At forming step 602,the plurality of field effect transistor (FET) layers 104 are formedover a substrate 103. The FET layers 104 can be formed by successivelydoping the substrate 103 material to produce alternating n-type andp-type semiconductor layers. The FET layers 104 may be epitaxiallyformed. After forming step 602, the method proceeds to forming step 604.

At forming step 604, the seed layer 109 is formed over the FET layers104. As mentioned above, the seed layer 109 is configured to promotemagnetic anisotropy for the memory element layers and provide ohmiccontact between the FET 102 and the memory element 108. To fulfill thefunctions of seed layers it can be made of many sub-layers, some ofwhich can be atomically thick. For example, a first sub-layer canprovide ohmic contact, a second sub-layer can compensate latticemismatch and the third sub-layer can promote perpendicular anisotropy infree layer.

Additionally, between these layers may be several monolayers of othermaterials to promote layer by layer growth of the whole structure. Afterforming step 604, the method proceeds to forming step 606.

At forming step 606, the free-magnetic layer 110 is epitaxially grownover the seed layer 109. Epitaxial growth may be achieved throughvarious techniques including variations of chemical vapor deposition(CVD) or atomic layer deposition (ALD). After forming step 606, themethod proceeds to forming step 608. At forming step 608, the tunnelbarrier 112 is epitaxially grown over the free-magnetic layer 110. Asmentioned above, the tunnel barrier can be comprised of insulating orsemiconducting materials. In order to compensate for mismatch betweenthe crystal lattices of the free-magnetic layer 110 and tunnel barrier112 and to enhance tunnel magneto resistance, sub-nanometer thickamorphous CoFeB layers can be used on both sides of the tunnel barrier112. After forming step 608, the method proceeds to forming step 610. Atforming step 610, the fixed-magnetic layer 114 is epitaxially grown overthe tunnel barrier 112. In this embodiment, the metallic cap layer 115is formed over the fixed-magnetic layer 114. After forming step 610, themethod proceeds to forming step 612.

At forming step 612, a mask layer is formed over the memory elementlayers. In this embodiment, the mask layer is formed directly over thefixed-magnetic layer 114. The mask layer is patterned as an array of“dots” to define the memory cell pillars. One skilled in the art wouldrecognize that the mask layer may be formed utilizing many differenttechniques, including conductive and non-conductive hard masks. Afterforming step 612, the method proceeds to etching step 614.

At etching step 614, portions of the memory element layers are etched todefine the memory elements 108. The mask layer may be removed after theetch process. After forming step 614, the method proceeds to formingstep 616.

At forming step 616, the encapsulation layer 204 is formed surroundingthe memory elements 108. The encapsulation layer 204 may be comprisedfrom a dielectric material. In some embodiments, the encapsulation layer204 is conformally deposited and directionally etched. As such, theencapsulation layer 204 may be designed as an encapsulation layer,protecting the sidewalls of the memory elements 108 during thesubsequent etches. Additionally, the encapsulation layer 104 may beconfigured to decrease the capacitance between the gate conductors 106and the memory elements 108. The encapsulation layer 204 may also beutilized as a mask to produce FETs 102 with diameters substantiallylarger than the memory elements 108. After forming step 616, the methodproceeds to etching step 618.

At etching step 618, portions of the FET layers 104 are etched to definethe FETs 102 and memory cell pillars. After etching step 618, the methodproceeds to forming step 702.

At forming step 702, the gate conductors 106 are formed along a firstaxis, aligned in parallel. The gate conductors 106 are self-aligned tolaterally surround a plurality of FETs 102 along the first axis. Afterforming step 702, the method proceeds to forming step 704.

At forming step 704, a filler layer 118 is formed as an insulatorbetween the memory cells 101. After forming step 704, the methodproceeds to forming step 706. At forming step 706, the bit lines 116 areformed along the second axis, in parallel. One skilled in the art wouldrecognize that many techniques may be utilized to form the bit lines,including damascene and subtractive etch methods. The bit lines 116 eachelectrically coupled to a plurality of memory elements 108 along thesecond axis. As mentioned above, the second axis is perpendicular to thefirst axis. After forming step 706, the method is complete.

FIG. 8 is a flowchart depicting different methods for forming the gateconductors 106, in accordance to embodiments of the present invention.The flowcharts represent sub-steps of forming step 702. The methodscontinue from etching step 618 and begin with forming step 801. Atforming step 801, a gate oxide and gate conductor layer is formed overand around the memory cell pillars. The gate conductor layer isplanarized and recessed below the memory element 108 height. Afterforming step 801, the method may proceed to forming steps 802, 812, or814.

In the case of the embodiment depicted in FIG. 3, forming step 801proceeds to forming step 802. At forming step 802, a gate photoresistmask is formed over the gate conductor layer in a line-space pattern.After forming step 802, the method proceeds to etching step 804. Atetching step 804, the line-space pattern is etched into the gateconductor layer, defining each of the gate conductors 106 as a rectanglewith a plurality of holes surrounding the memory cells 101 along thefirst axis. After etching step 804, the method proceeds to forming step708.

In the case of the embodiment depicted in FIG. 4, forming step 801proceeds to forming step 812. At forming step 812, a spacer mask isformed over the gate conductor layer in a fused-circle pattern. Afterforming step 812, the method proceeds to etching step 814. At etchingstep 814, the fused-circle pattern is etched into the gate conductorlayer, defining each of the gate conductors 106 as a plurality ofoverlapping rings surrounding the memory cells 101 along the first axis.After etching step 814, the method proceeds to forming step 708. Itshould be noted that a similar embodiment can be achieved by utilizing agate conductor spacer around the FETs 102.

In the case of the embodiment depicted in FIG. 5, forming step 801proceeds to forming step 822. At forming step 822, a spacer mask isformed over the gate conductor layer in a spaced-circle pattern. Afterforming step 822, the method proceeds to forming step 824. At formingstep 824, a gate photoresist mask is formed in a line-space pattern.After forming step 824, the method proceeds to etching step 826. Atetching step 826, the spaced-circle pattern is etched into the gateconductor layer, defining each of the gate conductors 106 is defined asa plurality of alternating ring and rectangular portions along the firstaxis, each of the ring portions surround one of the memory cells 101.After etching step 826, the method proceeds to forming step 708.

FIG. 9 depicts an intermediary step during fabrication of a memory arraydevice, in accordance with one embodiment of the present invention. FIG.9 shows the memory array device after forming step 612. As depicted, alllayers formed at this point are unpatterned and are each substantiallyplanar. As such epitaxial growth of the memory element 108 layersresults in higher quality memory elements 108.

FIG. 10 depicts an intermediary step during fabrication of a memoryarray device, in accordance with one embodiment of the presentinvention. FIG. 10 shows the memory array device after etching step 618.The figure displays the memory cell pillars 1002 resulting from etchingthe FET layers 104.

FIG. 11 depicts an intermediary step during fabrication of a memoryarray device, in accordance with one embodiment of the presentinvention. FIG. 11 shows the memory array device after forming step 702.

The flowcharts and diagrams in the Figures illustrate the architecture,functionality, and fabrication of possible implementations of a memoryarray device according to various embodiments of the present invention.It should be noted that, in some alternative implementations, thefabrication steps depicted in the flowchart and description may occurout of the order noted, depending upon the functionality involved.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method for fabricating a memory array devicecomprising: forming a plurality of field effect transistor (FET) layersover a substrate; forming a plurality of memory element layers over theplurality of FET layers, wherein each of the memory element layers isepitaxially grown; forming a plurality of memory cell pillars by etchingthrough portions of the plurality of memory element layers and portionsof the plurality of FET layers, such that each memory cell pillarincludes a FET and a memory element; forming a plurality of gateconductors configured along a first axis in parallel, wherein each gateconductor laterally surrounds a plurality of FETs along the first axis;and forming a plurality of bit lines configured along a second axis inparallel, wherein each bit line is electrically coupled to a pluralityof memory elements along the second axis, the second axis beingperpendicular to the first axis.
 2. The method of claim 1, whereinforming the plurality of memory element layers includes: forming a seedlayer over the FET layer; epitaxially growing a free-magnetic layer overthe seed layer, the free-magnetic layer including a ferromagneticmaterial; epitaxially growing a tunnel barrier over the free-magneticlayer, the tunnel barrier including a multiferroic material, wherein thetunnel barrier being situated between the free-magnetic andfixed-magnetic layers; and epitaxially growing a fixed-magnetic layerover the tunnel barrier, the fixed-magnetic layer including at least oneof a ferromagnetic material and an antiferromagnetic material.
 3. Themethod of claim 1, wherein forming the plurality of memory cell pillarsfurther comprises: patterning a mask layer over the memory elementlayers; etching the portions of the plurality of memory element layersto form the memory elements; forming an encapsulation layer surroundingthe memory elements; and etching the portions of the FET layers to formthe FETs, such that the diameters of the FETs are substantially greaterthan the diameters of the memory elements.
 4. The method of claim 1,wherein forming the plurality of gate conductors further comprises:depositing a gate conductor layer over and between the plurality ofmemory cell pillars; planarizing the gate conductor layer; recessing thegate conductor layer; and forming a gate photoresist mask in aline-space pattern along the first axis; etching the line-space patternthrough the gate conductor layer to form the plurality of gateconductors, such that each gate conductor is defined as a rectangle witha plurality of holes surrounding the memory cells along the first axis.5. The method of claim 1, wherein forming the plurality of gateconductors further comprises: depositing a gate conductor layer over andbetween the plurality of memory cell pillars; planarizing the gateconductor layer; and recessing the gate conductor layer; and forming aspacer mask laterally surrounding the memory elements in a fused-circlepattern along the first axis; etching the fused-circle pattern throughthe gate conductor layer to form the plurality of gate conductors, suchthat each of the gate conductors is defined as overlapping ringssurrounding the memory cells along the first axis.
 6. The method ofclaim 1, wherein forming the plurality of gate conductors furthercomprises: depositing a gate conductor layer over and between theplurality of memory cell pillars; planarizing the gate conductor layer;and recessing the gate conductor layer; and forming a spacer masklaterally surrounding the memory elements in a spaced-circle patternalong the first axis; forming a gate photoresist mask in a line-spacepattern along the first axis; etching the spaced-circle pattern throughthe gate conductor layer to form the plurality of gate conductors, suchthat each of the gate conductors is defined as alternating ring portionsand rectangular portions along the first axis, each of the ring portionssurround one of the memory cells.
 7. The method of claim 1, wherein theFET layers are epitaxially grown.